1. Field of the Invention
The present invention relates to an image forming apparatus and an image forming method in which an electrifying bias applied to electrifying means electrifies a surface of a photosensitive member, an electrostatic latent image is thereafter formed on the surface of the photosensitive member, and a development bias is thereafter applied to developer means so that a toner visualizes the electrostatic latent image into a toner image.
2. Description of the Related Art
This type of an image forming apparatus often sees a change in an image density due to the following factors: fatigue, degradation with age or the like of a photosensitive member and a toner; a change in a temperature, a humidity or the like around the apparatus; and other causes. Noting this, a number of techniques have been proposed which aim at stabilizing an image density through appropriate adjustment of a density control factor such as an electrifying bias, a development bias, a light exposure dose, etc. For example, the invention described in the Japanese Patent Application Laid-Open Gazette No. 10-239924 requires to properly adjust an electrifying bias and a development bias in an effort to stabilize an image density. That is, according to this conventional technique, reference patch images are formed on a photosensitive member while changing an electrifying bias and/or a development bias and an image density of each reference patch is detected. An optimal electrifying bias and an optimal development bias are thereafter determined based on the detected image densities, and a density of a toner image is accordingly adjusted.
The density adjustment is executed at the following timing. Specifically, after turning on a main power source of the image forming apparatus, a density is adjusted upon arriving at a state where the apparatus is ready to form an image, which is when a fixing temperature reaches a predetermined temperature or immediately after that, for example. Where a timer is built within the image forming apparatus, the density adjustment is executed at regular intervals, e.g., for every two hours.
By the way, while an electrifying bias and a development bias change in accordance with fatigue, degradation with age or the like of a photosensitive member and a toner, etc., the changes possess a continuity to a certain extent. Hence, when repeated density adjustment is desired, if a density is adjusted using an optimal electrifying bias and an optimal development bias which are obtainable from immediately preceding density adjustment, the current density adjustment is expectedly more accurate.
However, according to this conventional technique, a density is adjusted uniformly at the timing described above. More precisely, an electrifying bias-development bias characteristic is identified in advance, and three combinations of an electrifying bias and a development bias which satisfy the identified characteristic are registered in a ROM. Following this, for density adjustment, three reference patch images are formed using the different registered bias. Thus, the conventional apparatus executed the density adjustment without considering a result of immediately preceding density adjustment at all. Densities of the respective patch images formed in this manner are measured, and an optimal electrifying bias and an optimal development bias are determined based on the measured image densities.
Hence, there is a room in the conventional technique for improving an accuracy of calculation of an optimal electrifying bias and an optimal development bias, with respect to adjustment of an image density of a toner image to a target density by means of control of an electrifying bias and a development bias.
The present invention aims at providing an image forming apparatus and an image forming method which more accurately calculate an optimal electrifying bias and an optimal development bias which are necessary for adjusting of an image density of a toner image to a target density and accordingly stabilize the image density.
In fulfillment of the foregoing object, an image forming apparatus and method are provided and are particularly well suited to density adjustment of a toner image based on image densities of a plurality of patch images. Control means adjusts an image density of the toner image to a target density by controlling an electrifying bias and a development bias. Every time adjustment is finished, the control means causes memory means to store the electrifying bias and the development bias which are obtained after the adjustment as an optimal electrifying bias and an optimal development bias, respectively. When adjustment is performed repeatedly, based on the optimal electrifying bias and the optimal development bias stored in the memory means, the control means controls such that a plurality of patch images are formed in sequence while changing at least one of the electrifying bias and the development bias.
An image forming apparatus and method according to the present invention will be described in derail by means of the following most suitable embodiments thereof.